Gamma-Ray Detectability and Viewing Geometry of …tauris/NS2014/Guillemot_Gamma-ray-MSPs.… ·...

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Gamma-Ray Detectability and Viewing Geometry of MSPs Lucas Guillemot 5 th BONN workshop on the Formation and Evolution of Neutron Stars MPIfR / AIfA Bonn Uni., 05/03/14 (based on Guillemot & Tauris, MNRAS in press, 2014)

Transcript of Gamma-Ray Detectability and Viewing Geometry of …tauris/NS2014/Guillemot_Gamma-ray-MSPs.… ·...

Page 1: Gamma-Ray Detectability and Viewing Geometry of …tauris/NS2014/Guillemot_Gamma-ray-MSPs.… · Gamma-Ray Detectability and Viewing Geometry of MSPs Lucas Guillemot !! 5th BONN workshop

Gamma-Ray Detectability and Viewing Geometry of MSPs

Lucas Guillemot !!

5th BONN workshop on the Formation and Evolution of Neutron Stars !

MPIfR / AIfA Bonn Uni., 05/03/14

(based on Guillemot & Tauris, MNRAS in press, 2014)

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Introduction

The Large Area Telescope on Fermi (launched in June 2008) has detected pulsed γ-ray emission from 147 pulsars.

About 40% (62) are MSPs! (see https://confluence.slac.stanford.edu/display/GLAMCOG/Public+List

+of+LAT-Detected+Gamma-Ray+Pulsars)

!Detected γ-ray pulsars have the largest values of

Ė (= 4π2I Ṗ/P3) / d2.

!Much larger fraction of MSPs with high values of Ė / d2 are

detected in γ rays, compared to the normal pulsar population.

!Nevertheless, some high Ė / d2 MSPs escape detection in γ

rays. Possible explanations: • underestimated distance (especially if based on

NE2001), • unfavorable orientation, leading to γ-ray beams not

crossing our line of sight.

!2

HTRU – X. Timing solutions for 16 MSPs 17

Table 8. Gamma-ray emission properties of PSRs J1125−5825, J1446−4701, J1543−5149 and J1811−2405. The weighted H-test parameterswere calculated by selecting photons found within 5◦ of the pulsars, with energies larger than 0.1 GeV and weights larger than 0.01. See Fig. 9for the corresponding gamma-ray light curves under the same selection cuts. Details on the measurement of the spectral parameters can befound in Section 4.10.

Parameter J1125−5825 J1446−4701 J1543−5149 J1811−2405

Weighted H-test 100.7 165.4 65.1 37.9Spectral index, ! 1.6 ± 0.5 1.3 ± 0.4 2.3 ± 0.3 1.6 ± 0.4Cutoff energy, Ec (GeV) 8 ± 7 4 ± 2 6 ± 3 3 ± 2Photon flux above 100 MeV, F100 (10−8 cm−2 s−1) 0.8 ± 0.7 0.6 ± 0.2 5.4 ± 0.4 2 ± 2Energy flux above 100 MeV, G100 (1011 erg cm−2 s−1) 0.9 ± 0.3 0.7 ± 0.1 2.4 ± 0.2 1.4 ± 0.8Luminosity, Lγ = 4πG100d

2 (1033 erg s−1) 7.1 ± 2.4 1.9 ± 0.3 17 ± 1 5.5 ± 3.1Efficiency, η = Lγ /E 0.09 ± 0.03 0.05 ± 0.01 0.23 ± 0.02 0.2 ± 0.1

Figure 9. Radio and gamma-ray light curves for the four MSPs in our sample with Fermi LAT detections. Two pulsar cycles are shown for clarity. The radioprofiles are based on 1.4 GHz observations conducted at Parkes, while the gamma-ray profiles were obtained by selecting Fermi LAT photons with reconstructeddirections found within 5◦ of the MSPs, and with energies larger than 0.1 GeV. The photons were weighted by the probability that they originate from thepulsars as described in e.g. Kerr (2011). Photons with weights smaller than 0.01 were rejected. Horizontal dashed lines show the estimated background levels,obtained by following the method described in Guillemot et al. (2012). The grey shaded regions indicate the OFF-pulse intervals used for the spectral analysespresented in Section 4.10, the ON-pulse regions being defined as the complementary intervals.

expected energy fluxes for these pulsars much smaller than thelowest value reported in Abdo et al. (2013) for an MSP, because ofthe generally large distance values, with the notable exception ofPSR J1731−1438. The latter MSP may be inefficient at converting

its spin-down power into gamma-ray emission, or its gamma-raybeams may not cross the Earth’s line of sight. The high-E but distantMSPs in this sample could contribute to the diffuse emission seenby the Fermi LAT around the Galactic plane.

at MPI R

adio Astronom

y on February 25, 2014http://m

nras.oxfordjournals.org/D

ownloaded from

LAT (blue, E > 0.1 GeV) and Parkes 1.4 GHz (red) light curves for PSR J1811-2405.

!From Ng et al., MNRAS in press (2014).

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γ-ray emission from MSPs

!3

814 VENTER, HARDING, & GUILLEMOT Vol. 707

Figure 14. Sample light curves for an OG1 model with P = 2 ms.(A color version of this figure is available in the online journal.) Increasing viewing angle ζ

Incr

easi

ng m

agne

tic in

clin

atio

n α

Figure adapted from Venter et al. (2009). !

Here: plots of the predicted radio and γ-ray emission of MSPs under the Outer

Gap model. !

Configurations with low ζ values and/or low α values are not detectable

(no γ-ray emission at all, or very weak modulation of the signal).

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Can we constrain the geometry of MSPs?

Methods for extracting pulsar geometry angles (namely, magnetic inclination α and viewing angle ζ):

• fits of radio polarization (Radhakrishnan & Cooke 1969). Generally does not work for MSPs…

• radio and γ-ray light curve modeling (e.g. Johnson 2011), for γ-ray-detected MSPs only!

• our approach: binary evolution.

!Key arguments:

• MSPs originate from LMXBs. The long time-scale (108 - 109 yrs) of mass transfer in LMXBs should cause the spin axis to align with the orbital angular momentum

vector: ζ = i. • mass function: relation between the companion mass,

the pulsar mass, the inclination i. • How to calculate the companion mass? Tauris & Savonije

(1999): unique relationship between Porb and MWD for MSPs orbiting He WD with 0.13 < MWD / M⊙ < 0.46.

!4

radio emission cone

γ-ray emission fan beam

f(MNS

,MWD

, i) =4⇡2

G

(ap sin i)3

P 2

orb

=(M

WD

sin i)3

(MNS

+MWD

)2

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MWD - Porb relation (TS99)

!5

Adapted from Tauris and van den Heuvel, ApJ Lett. (2014).

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Predicting viewing angles for a sample of MSPs

We selected MSPs (P < 30 ms) in binary systems likely to be orbiting He WD companions (`BinComp’ parameter in the ATNF pulsar catalog), completed with other MSPs with ζ

constraints from light curve modeling (Johnson 2011) or sin(i) measurements.

!For each MSP we calculated Ė / d2, accounting for the Shklovskii effect whenever possible.

!Available sin(i) and MNS values were compiled. If MNS not known, assumed 1.53 +/- 0.21 M⊙

(average observed for MSPs with He WD companions, see Tauris et al. 2012).

!Viewing angles ζ were finally assigned to the MSPs in the sample in this order:

• for MSPs with ζ constraints from light curve modeling, the best-fitting value was used. • if not, but constraints on sin(i) exist, use ζ = i. • if none of the above, use ζ = i calculated using the TS99 relation.

!⇒ Obtained a sample of 70 MSPs with ζ estimates.

!6

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Example for PSR J0218+4232

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Example for PSR J0218+4232

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TS99: MWD = 0.207 - 0.225 M⊙. In line with Bassa et al.

(2003) (~0.2 M⊙).

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Example for PSR J0218+4232

!7

Predicted ζ value: 58+14-10 deg.

TS99: MWD = 0.207 - 0.225 M⊙. In line with Bassa et al.

(2003) (~0.2 M⊙).

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Predicted vs measured ζ values

!8

ζ from light curve modeling analyses.

ζ from sin(i) measurements or TS99 if not available.

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Predicted vs measured ζ values

!8

ζ from light curve modeling analyses.

ζ from sin(i) measurements or TS99 if not available.

Good overall agreement between ζPredicted and ζLC Modeling!

!Potential outlier: J0218+4232, with ζLC

Modeling = 8°. Would imply MWD = 1.9 M⊙!

!Alternative solution: ζLC Modeling = 32°.

!Average difference: 8° with an rms of 8°.

Spearman coefficient:

0.88 ~ 1 (p-value = 8e-5!).

!Our procedure provides a reliable

method for estimating ζ.

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Gamma-ray detectability metric

!9

green stars: gamma-ray MSPs.

red circles: MSPs undetected in gamma rays.

filled symbols: Ė corrected for the Shklovskii effect.

!γ-ray-detected MSPs occupy the upper part

of the plot:

⇒ Ė / d2 is a good measure of γ-ray

detectability.

!Define two samples:

• Ė / d2 > 1.5e34: 75% of MSPs are detected in γ rays.

!• Ė / d2 > 8e32: 50% of MSPs are detected

in γ rays.

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Viewing angle distribution

!10

Distributions of the viewing angle ζ for the two pulsar samples.

MSPs not detected in γ rays appear to be distributed towards smaller ζ values on average!

!KS tests: p-value of 3% for both samples. Marginally significant effect, but we postulate that

low ζ values are at least partly responsible for the non-detection of high Ė / d2 MSPs.

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Conclusions

The viewing angles of binary MSPs are well described by the orbital inclination angles,

confirming that spin axes align with the orbital momentum vector during recycling.

!We find evidence for slightly different viewing

angle distributions for γ-detected and undetected MSPs: unfavorable orientations can

prevent us from seeing γ-ray pulsations!

!New Pass8 data: the LAT’s sensitivity to

pulsed γ-ray emission has increased substantially.

!Stay tuned for more MSP detections in γ rays!

!11